The present invention relates generally to electrical wet shaving devices. More particularly, the present invention relates to battery powered, oscillating razors in which an internally disposed eccentric mass is rapidly rotated to forcibly vibrate the shaver cutting blade or blades.
It will be well appreciated that a comfortable shave is highly desirable. The prior art suggests that the employment of a rotating mass within a hand-held wet shaver has been found desirable to increase shaving comfort. A plurality of prior art shavers exist in which an internal battery is disposed for rotating a motor-driven eccentric to produce vibration.
One of the most relevant prior art U.S. patent references known to us disclosing the broad teachings of this general concept is seen in U.S. Pat. No. 2,054,287, issued to Bohm, in Sept. 1936. Bradshaw in U.S. Pat. No. 860,849 discloses a cam and spindle mechanism for reciprocating a cutting blade. Bohm suggests a safety razor having an internal eccentric driven by an associated motor. Similar prior art U.S. patents include Tiffin, No. 3,636,627; Douglas, No. 3,772,779; Alexander, No. 3,611,568, and Jaffe, No. 2,112,402. The latter reference includes electrical and mechanical structure driving and rotating an oscillating mass for producing vibration. U.S. Pat. No. 1,798,831 issued to Brazeal discloses a safety razor having means for producing movement on a diagonal path of travel to produce a shear cut in an effort to promote efficiency. U.S. Pat. No. 2,552,688 issued to H. F. Partridge discloses a vibrating safety razor which includes a lever and crank apparatus for reciprocating the razor and blade in short, swift strokes to effect a lateral vibration of the blade as it is drawn over the face of a user. This patent differs from the Bradshaw and Brazeal patents in that it is drawn to a device which effects motion to both the razor and the blade, rather than to the blade and blade head only.
Most of these patents exhibit a means for vibrating a cutting surface during wet shaving in which an internal vibratory system is employed to rapidly oscillate the cutting surface. Jaffe discloses a wind-up spring-operated motor drives a rotatable eccentric including a spindle carrying a rotatable pendulum. Alexander discloses a basic system in which a battery spaced apart from a motor drives a rotatable eccentric to provide vibration. A similar prior art approach is exhibited by Tiffin.
Other prior art patents of possible relevance comprise Design Pat. Nos. 150,439; D211,553; D161,675; and D254,209. The prior art structure of U.S. Design Pat. No. D279,930, was invented previously by one of the same inventors as in the instant case. Also of possible relevance are U.S. Utility Pat. Nos. 2,904,883; 3,610,080; 1,719,827; 3,131,974; 2,230,630; 2,227,996; 2,609,602; 2,423,595; 3,038,254.
All of the relevant prior art oscillating wet shaver references known to us disclose structure including a housing having a longitudinal axis which is substantially coincident with the axis of mass of the internally disposed battery power supply. Such devices do not exhibit sufficient dynamic coupling (i.e. "roll-couple") as does the present invention. Moreover, the prior art teaches that rotational velocity of the motor, depending on the dampening qualities of the device, should be effectuated so as to produce operation at a point which is most electrically efficient. As will be appreciated by those familar with mechanical engineering resonance concepts, devices, such as the razors aforedescribed, exhibit a characteristic mechanical period of vibration, and thus are characterized by a resonance.
In mechanical systems having one degree of freedom which are subjected to forced, dampened vibration, the ratio of transmitted force to impressed force follows a well-defined and widely discussed relationship with applied frequency. The best understood characteristic is operation at the "critical speed" of the system wherein transmitted forces peak at relatively high multiples of impressed force. The frequency ratio at the critical speed is 1, by definition. A less widely-understood characteristic is system behavior at frequencies above 1.414 (i.e., the square root of 2) in which a not insubstantial degree of dampening may cause the transmitted force to be very high multiple of impressed force. Critical speed phenomena will therefore be observed in only a very narrow speed range, wherein the latter transmitted force ratio continues to increase with increasing speed in a predictable manner.
For purposes of energy efficiency prior art devices broadly relate the frequency of eccentric oscillation to correspond generally with the characteristic resonance of the device. Thus the "frequency ratio" which relates the motor speed to the characteristic mechanical resonance of the device, has been as close as possible to unity (i.e. 1) in prior art devices. As will be recognized by those skilled in the mechanical engineering arts, frequency ratios in dampened oscillating systems can be graphically related to such characteristic phenomena as transmissibility, which may be broadly defined as the ratio of resulting force to impressed force. A graph plotting frequency ratio against transmissibility illustrates the effects of dampening. Speed ratio and resonance are related to the concept of dampening in a mechanical, vibratory system. Such curves are used to characterize dampened vibrating mechanical systems, and it has previously been thought that eccentrically vibrating razors should operate at a motor speed relatively close to that rotational frequency which corresponds to a maximum transmissibility.
We have recognized the desirability of providing a vibratory razor system which effectuates a frequency ratio above 1.414. Means have been provided, therefore, to effectuate an observed frequency ratio above this "square root of two" limit. Moreover, due to the location of the center of gravity and its offset from the longitudinal center of mass and the axis of the handle, we have determined that a better cutting phenomena occurs through the concept of roll-couple. In other words, we have found it desirable to provide a microscopic, orbital motion upon the cutting edges of the razor which do not exhibit a uniform orbit across the plane of the cutting blades.
It is also extremely important to provide a razor which is relative mechanically insensitive to the manner in which it is held. An effective shaver will encounter a material (e.g., hair) of constantly changing properties. Not only is one person's hair different from another's, the hair on the chin is different from hair on the cheek, and the differences change with time. The frequency applied must take advantage of the skin's normal spring-dampening characteristics to insure vibration above the system's critical frequency. Unless the rotational speed and internal construction of vibratory razor devices are appropriately designed, the force transmittal characteristics and hence the shaving efficiency and comfort, will be deleteriously affected in response to the manner in which the razor is gripped or held during shaving. Hence it is desirable to provide a razor of the general characteristics described which is relatively insensitive to the manner in which it is held by the user.
Thus prior art oscillating razor references have previously employed a rotation speed which apparently maximizes energy efficiency and effectuates the widest possible magnitude of blade displacement, operating at a frequency ratio very close to unity. Among other things, we have discovered that for purposes of maximizing comfort it is desirable for an oscillating razor to operate at a frequency ratio of between 1.414 and 2.0, notwithstanding the fact that theoretical electrical motor efficiency may be sacrificed somewhat. Shaving comfort is related more to transmitted force than to amplitude. When motor speeds exceed the second harmonic (i.e. twice the natural resonant frequency of the razor device when hand held), comfort is reduced. We have also determined that it is desirable for such a device to exhibit a roll couple.